Measurement of radioactivity in soils of Karamjal and Harbaria mangrove forest of Sundarbans for establishment of radiological database

This work presents the first in-depth study of soil radioactivity in the mangrove forest of Bangladesh part of the Sundarbans. It used HPGe gamma-ray spectrometry to measure the amount of natural radioactivity in soil samples from Karamjal and Harbaria sites of the world’s largest mangrove forest. The activity concentrations of most of the 226Ra (14±2 Bqkg-1 to 35±4 Bqkg-1) and 232Th (30±5 Bqkg-1 to 50±9 Bqkg-1) lie within the world average values, but the 40K concentration (370± 44 Bqkg-1 to 660±72 Bqkg-1) was found to have exceeded the world average value. The evaluation of radiological hazard parameters revealed that the outdoor absorbed dose rate (maximum 73.25 nGyh-1) and outdoor annual effective dose (maximum 0.09 mSvy-1) for most samples exceeded the corresponding world average values. The elevated concentration of 40K is mainly due to the salinity intrusion, usage of fertilizers and agricultural runoff, and migration of waste effluents along the riverbanks. Being the pioneering comprehensive research on the Bangladesh side of the Sundarbans, this study forms a baseline radioactivity for the Sundarbans before the commissioning of the Rooppur Nuclear Power Plant in Bangladesh.

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Enter: The author(s) received no specific funding for this work.Keywords: Mangrove forest, The Sundarbans, HPGe, Natural radioactivity, Effective Dose

1.Introduction
.The Sundarbans provide a land-sea interface and seawater-freshwater interface that is home to a diverse ecosystem and provides a unique connection of different atmospheres, geologies, lithosphere and hydrosphere.It has been reported that The Sundarbans is affected by highly diverse agricultural and aquaculture activities which involve the usage of fertilizers and other chemicals, as well as soil erosion and wastewater runoff into the Sundarbans from these sites (4).Since The Sundarbans is a vital site for agriculture and fishing, raised levels of radioactivity may cause by the influx of fertilizers and chemical pollution which might affect the thousands of animal species living in the forest, and the bio-magnification of radioactivity may also affect the human population via the food chain.In the Indian region of The Sundarbans, elevated levels of 40 K were reported in its coastal zone (5).This calls for a similar study to measure the radioactivity in the Sundarbans of Bangladesh as the inflicting hazards are the same.
Moreover, Southern Bangladesh is experiencing major industrial developments including the construction of 9 power plants.Of these, 6 projects are located in 3 locations surrounding The Sundarbans.Among these power plants, the Rampal Coal-based Thermal Power Plant poses the greatest concern, as it is located only 4km outside of the Ecologically Critical Area (ECA) on the east bank of the Pashur River.Furthermore, the Pashur River will be used for the transportation and trans-shipments of coal for the Rampal Power Plant, which poses the risk of coal spillage in the river.It has also been reported that 1-2 annual shipping accidents are occurring in The Sundarbans which mainly involve coal and fertilizer shipping (6).In addition, The Rooppur Nuclear Power Plant is due to commissioning in 2023-24, which will use the Padma River water as its tertiary coolant.Since The Sundarbans lies at the delta of Padma River, the releases, if any, from the Power Plant might affect the overall radioactivity level of the waterbodies, sand, and sediments of The Sundarbans.Considering all of these foreseeable events, it is necessary to obtain the baseline data of NORMs in The Sundarbans areas, to monitor any radiological changes in future due to the natural or anthropogenic activities.
The Sundarbans, being a World Heritage Site, attracts thousands of local and foreign tourists every year (7).This also raises concerns that the radioactivity that the tourist is exposed to is unknown.A few studies were reported on radiation levels of the mangrove forests in Brazil (8) and petrified wood forestsin Egypt (9), and three forests in Norway (10).Several studies have been conducted to measure the radioactivity levels in the mangrove forest of the Indian Sundarbans (1,5), ( 11), ( 12), (13).Only two studies have been found to date in The Sundarbans of Bangladesh site, one of which reported the NORM distribution in sediment samples (14) and the other assessed the number of trace elements in sediments samples (15); but no systematic study has been carried out to date, to the best of our knowledge, to measure the radioactivity levels of tourist spots in the The Sundarbans.Moreover, this study includes the Karamjal and Harbaria Eco-Tourism Centres, visited by more than 200,000 tourists annually, which were never studied before.Therefore, this present study aims to measure the concentration of NORMs present in the soil samples of Sundarbans, the first of its kind, to check if any artificially occurring 137 Cs is present, and to evaluate the associated radiation risks to the tourists and the local dwellers who are dependent on the Sundarbans economically, aqua-culturally and agriculturally, and diverse wildlife inhabiting the forest.This study also aims to provide baseline data which is important due to the recent commissioning of a nuclear power plant and several thermal power plants surrounding the Sundarbans.

Study Area
Physiography, Geology, Geomorphology, Pedology: The area is located in the southwestern part of the deltaic coastal plain region of Bangladesh.Physiographically, the mapped area (
(1) Acid Sulfate Soil: Grey or dark grey clayey soils with yellow patches develop in tidal deposit areas once covered by mangrove vegetation.These soils are usually saline in dry seasons.
(2) Peat soils: It is highly organic dark colored soils developed under submerged conditions, mainly in the basin areas and other smaller waterlogged bells and valleys.Suppose drained and allowed to dry out, peat shrinks irreversibly, thus causing cracking and subsidence of the ground surface.

Sampling and preparation procedure
A total of 30 soil samples were collected from the area around the Sundarbans (15 from Karamjal and 15 from Harbaria), down to a depth of 10 cm during December 2021, and a global positioning system was used to record each location.The samples were collected following the systematic random sampling technique (19).
After removing extraneous components like roots, pebbles, and plant matter, along with other impurities, the samples were thoroughly mixed.Each sample, weighing between 0.5 and 1 kg, then immediately stored in airtight, clean zip-lock polyethylene bags, adequately labeled.The samples were transported to the sample preparation room of the Health Physics Division of Atomic Energy Centre Dhaka (AECD) for further processing.The samples were homogenized, weighed, and dried to reduce moisture content in a temperature-controlled furnace.All samples were then put into radon-impermeable, airtight plastic cans after being crushed and powdered.
Then they were kept for at least 30 days to reach secular equilibrium between the yields of the radioactive elements 222 Rn, 220 Rn, and their transient daughter elements, 226 Ra, and 224 Ra.Cross contamination was carefully avoided throughout sample preparation, measurement, and sampling (20).

Measurement procedures and data analysis
Using a high-resolution coaxial HPGe gamma-ray spectrometer and the related electronics, the activity concentrations of gamma-ray releasing radionuclides within the samples were determined.
WhereA is the specific activity in Bqkg -1 ,cps is the count rate, ε is the HPGe detector's counting efficiency at the specific gamma-ray energy, ρ γ represents gamma-ray emission probability, and w is the sample weight in kilograms (kg).The minimal detectable activity concentration (MDAC) for the gamma-ray measurement system method was calculated using Eq (2) as stated in (20): where K is the statistical coverage factor, with a value of 1.64 (at the 95% confidence level), B is the number of background counts for the relevant radionuclide, T is the counting time, and I and w (in kg) have the same usual meaning as in Eq (1).The MDAs for 226 Ra, 232 Th, and 40 K were determined to be 0.35 Bq/kg, 0.64 Bq/kg, and 2.2 Bq/kg, respectively.
Using the uncertainty propagation law of the relevant quantities represented in Eq (2), the uncertainty of the measured radioactivity was determined.Eq (3) expressed the mathematical formulation for calculating the uncertainty of the determined radioactivity (24,25) Combined Standard Uncertainty =   × √( The sample counts, counting time, gamma-ray emission probability, sample weight, and counting efficiency are represented by the letters N, T, ρ γ , w, and ε, respectively.The calculateduncertainty of the relevant radionuclides varies from 5 to 10%.

Energy and efficiency calibration
The accuracy of the measured data largely depends on the energy and efficiency calibration of the detector, which must be carried out with extreme care.The detector's energy calibration was performed using common point sources like 22 Na, 57 Co, 60 Co, 133 Ba, 137 Cs, etc.The percentage of radiation quanta (particles or photons) that a detector can detect out of all the radiation quanta that a source emits is known as the detector's efficiency.A standard source was made by combining 152 Eu of known activity with Al2O3 matrix (26) and manufactured in the same containers as the samples to determine the detector efficiency.

Radiological hazard parameters
Radium equivalent activity: The external and internal dose from 222 Rn and its daughter are correlated by the radium equivalent activity, Raeq (Bqkg -1 ).The Raeq was determined using Eq (4) to compare the combined radiological effect of 226 Ra, 232 Th, and 40 K in the materials (27).For safe use, the maximum Raeq value must be lower than 370 Bqkg -1 .
CRa, CTh, and CK represent the mean specific activities of 226 Ra, 232 Th, and 40 K in Bqkg -1 , respectively.

The absorbed dose rate in air and annual effective dose evaluation:
The external absorbed dose rate, Dout, to the public's exposure due to the released gamma rays from the studied material at 1 m above the ground, was calculated using the following Eq (5) = 0.427  + 0.662 ℎ + 0.0432  (5) Dout represents the outside absorbed dose rate in (nGy/h) due to gamma-ray exposure, while the other symbols have their usual meaning.Because human beings spend much more time indoors than they do outside, indoor exposure becomes more significant.Beside this, earth crust-derived products such as brick, sand, cement, paints, tiles, etc. are widely utilized in the construction of dwellings, therefore, assessing the indoor exposure is critical, and Eq (6) is used to calculate it (28,29).
The measured indoor and outdoor exposures can be used to calculate the annual effective doses of Ein and Eout.To accomplish this, the absorbed dose rate in the air was converted to the effective dose received by an adult using a conversion factor of 0.7 Sv/Gy (20).Furthermore, because people spend roughly 80% of their time indoors and 20% outdoors, the values 0.8 and 0.2 for the indoor and outdoor occupancy factors are used to calculate the representing dosage.

External hazard (Hex) and internal hazard (Hin) indices evaluation:
Using the external and internal hazard indices, the permissible equivalent dose should be lined up with a restricted value.Building materials should have a value of Hex that is less than or equal to unity to reduce the radiation dosage (20).By using Eq (9) external hazard index (Hex) can be calculated (31).Bqkg -1 for 40 K, 232 Th and 226 Ra respectively (30) in the studied soil samples.The order of natural radionuclide activity concentration was 40 K> 232 Th> 226 Ra.In all terrestrial environments of the earth's crust, 40 K is naturally abundant (33)and is a well-known primary weathering product (34).

Results and Discussion
Due to its high water solubility, 226 Ra may experience surface run-off on muddy terrain, whereas 232 Th sticks to the soil more due to its limited geochemical mobility (35).Additionally, the lithosphere naturally contains three times as much thorium as uranium or radium.Each location in the world has different geological and topographical conditions, which affect radioactivity in soils (36)(37)(38).Variations in soil activity levels can be attributed to factors such as the soil-towater ratio, the rate and amount of rainfall, soil drainage, site characteristics, and other environmental variables like meteorological conditions, soil use patterns, fertilizer use, evaporation, etc. (39).Additionally, the radionuclides' chemical characteristics have a significant impact on how they migrate.The radioactivity levels of the examined soil are also influenced by the weathered components of the nearby deposited rocks (40).The specific levels depend on the type of rock the soil is made of.Igneous rocks, like granite, have higher radiation levels, while sedimentary rocks have lower levels (41)(42)(43)(44)(45)(46).Low levels of 226 Ra in the collected soil are due to the absence of interaction with igneous rocks and uranium-rich minerals such apatite, zircon, etc. (47).In soil that contains much monazite, the concentration of 232 Th is higher (48)(49)(50).
Comparatively low values of 232 Th in the current study indicate the absence of monazite bearing minerals.On the other side, no 137 Cs activity was found in the collected soil samples, ruling out the buildup of fallout radiation from nuclear accidents like Chernobyl and Fukushima.
Large portions of the Sundarbans have been recovered during the past few decades for use in agriculture and settlement (5,11).In some circumstances, fertilizers are frequently employed to enhance the nitrogen balance of soil for agronomic growth, which is a source of 40 K in the environment (5,11,51).The majority of the soils of the Sundarbans are heavy-textured, brownish/grayish black silty or clayey in composition, with the availability of swampy soils near the sea [21].The 40 K concentration of soils and sediments is typically high since it is indigenous to this coastal region without any such influence from fertilizers (52,53).Several cargo catastrophes involving coal fly ash, fertilizer, and oil have recently happened along the Sela and Poshur rivers in Bangladesh's Sundarbans (15).There are worries about the ecological catastrophe that occurred in the Sundarbans due to the Sela and Poshur rivers' connections to the tiny creeks there.The mangrove belt in the Sundarbans estuary is very dynamic, with consistent deposition and erosion of silt and sediment from various interconnected rivers, creeks, channels, etc. (1).The Mongla port city, which is close to Sundarbans, is home to a number of oil, petroleum, and cement factories.Therefore, throughout the Sundarbans estuary, there is a substantial risk of buildup from home and agricultural effluents, industrial trash, etc.Overall, it can be concluded that the Sundarbans have a high native 40 K content, and factors such as rising salinity, the use of fertilizers to boost crop yields, and the buildup of upstream waste and undesirable effluents along this coastal zone may have contributed to high 40 K content.
x 3 in.)   K on the mangroves, as well as the fact that the RFM location is surrounded by granites holding around 7% K2O (8).Nabanita Naskar et al. (1) explained that the possible reason for the high level of 40 K in Indian Sundarbans is that the tidal mangrove belt in the Indian Sundarbans is highly dynamic due to the constant deposition and erosion of silt, sediment from the many rivers, creeks, channels, etc. that flow into it.
In a fascinating comparison of pre-and post-tsunami radioactivity in sediment samples from the Pichavaram mangroves, Satheeshkumar et al. (13) found that the activity of 238 U and 232 Th had decreased from pre-tsunami data (25.9±15.8 Bqkg -1 and 65.1±34.5 Bqkg -1 ) to posttsunami data (12.2±4.2 Bqkg -1 and 11.7±5.0Bqkg -1 ).The hydrodynamics of the tsunami waves, which removed vast quantities of decades-old beach sediments, is a crucial factor in the low level of radioactivity in Pichavaram.In addition, it spread a substantial layer of black sand.As a result, background radiation levels in the environment are reduced (13).In the Krusadai Island Mangrove, Gulf of Mannar, India, low concentrations of 238 U, 232 Th, and 40 K were found, due to the low concentration of radiation-bearing minerals in the study area, as well as the coastal geography and the direction of the currents, the concentration of uranium-and thorium-bearing minerals is reduced (12).The Norwegian sites of Fen and Bolladalen had extremely high 232 Th concentrations [48] because the whole Fen Complex was an active volcano suggests that different ores were present at varying depths and that there were heterogeneous rocks present that included mixes of several minerals and different elements.On the other hand, Bolladalenswas seen as a representation of legacy NORM and unaltered 232 Th-rich sites, respectively, due to its complex mixture of rodbergite and rauhaugite.
The soil samples collected from three different depths (0-55, 5-15, and 15-30 cm) were analyzed to observe the horizontal as well as vertical distribution of the radioactivity concentration in the Bhawalgahr forest, Gazipur district, Bangladesh (54).The average activity concentrations of 226 Ra, 232 Th, and 40 K for all the samples were higher than those of the worldwide average values, which may be attributed to the area's geological characteristics.The vertical distribution of 232 Th and 40 K showed a decreasing tendency of activity concentration with depth, whereas no particular tendency with depth was observed for 226 Ra in that study.The results of the current study are in close agreement with earlier studies conducted globally due to the close geological coherence, such as in Bhawalgahr forest, Gazipur district, Bangladesh [49]; Shamnagarupazila, Satkhira, south-west of Sundarbans, Bangladesh [50]; Sundarbans Mangrove, Bangladesh [14]; and Sundarbans Mangrove; India [1].
The radium equivalent activity, absorbed dose rate (outdoor and indoor), external and internal hazard index, external and internal annual effective dose values are reported in Table 3, and the spatial distribution of different parameters in Harbaria and Karamjal are represented in Figures 3 and 4.   (30).Almost all the values (except Sample 3 in Karamjal) of indoor annual effective dose are within the worldwide average indoor annual effective dose, which is 0.41mSvy -1 (30).
As almost all the values of 40 K, and most of the outdoor absorbed dose rate in air and outdoor annual effective dose exceedthe corresponding population-weighted world average values, we can conclude that the study area is safe for short-term stay by the visitors but not radiologically safe (specially Harbaria) for long-term residence because the presence of such radionuclides at high concentrations can potentially result not only in harmful healtheffects, such as chronic lung diseases, anemia, and differentcancers but also in genetic mutations that affect not only humans but also the entire biota (38,58).

Conclusions
This study was carried out to evaluate the distribution of naturally occurring radioactive materials ( 226 Ra, 232 Th, and 40 K), as well as artificial radionuclides, and to calculate the associated radiological hazard parameters at Karamjal and Harbaria, the two primary tourist spots in the world's largest mangrove forest, Sundarbans.
The range of activity concentration of 226 Ra, 232 Th, and 40 K in Karamjal was 14-35 Bqkg -1 , 30-46 Bqkg -1 , and 370-660 Bqkg -1 , respectively.In the Harbaria region, the values were found to be 20-31 Bqkg -1 , 35-51 Bqkg -1 , and 510-580 Bqkg -1 for 226 Ra, 232 Th, and 40 K, respectively.Most samples contain a higher 40 K concentration than the population-weighted world average values, which is attributed to the natural abundance of potassium bearing minerals, weathering process, and fertilizer use etc.In regard to anthropogenic activity, no radioactivity of 137 Cs was detected in the collected soil samples.
This study observed that some of the outdoor annual effective dose values in Karamjal and nearly all outdoor annual effective doses in Harbaria are higher than the worldwide average outdoor effective dose.As a result, the residents have valid reason to be concerned because they spend a lot of time in this exposure environment, which eventually puts their health at risk from radiation.
A few recommendations are proposed by this current research:  A detailed survey is necessary for mapping the distribution of NORMs in the world's largest mangrove forest, Sundarbans, which will be an essential reference data.
 Frequent monitoring is needed to evaluate the effect of soon to be commissioned Rooppur Nuclear Power Plant in Bangladesh.
 The greater activity concentration of 40 K indicates the existence of potassium bearing minerals resources in the nearby areas.
 The outdoor effective dose in some regions is higher than the population-weighted average value, a significant concern for the local inhabitants, appropriate knowledge and mitigation strategies may need to be introduced.
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Figure 1 )
belongs to the complete physiographic units Ganges Tidal Floodplain and Sundarbans.The topography of the site is mainly flat with gentle relief, formed under fluvial-tidal conditions.Physiographically it belongs to the lower deltaic plain (Mangrove swamp deposit) unit (16).The area's sediments (Holocene terrain) consist mainly of an admixture of clay, silt, and fine sand.The covering morphostrtigraphic units are supratidal and intertidal deposits.The largest Mangrove forest (Sundarbans) in the World falls under the investigated area, specially Sarankhola Range and Chandpai Range.The site is gently sloping from high-level deltaic plains in the north to lowlevel deltaic plains in the south towards the Bay of Bengal.The area's elevation ranges from 0.5 to 3.79 m above mean sea level.The region experiences a humid tropical monsoon climate.The average rainfall in the location from 2006-2011 was about 2024.8 mm/year.

( 3 )
Calcareous Brown Floodplain Soils: These soils comprise brown loams to silty clay.They are grayish where transplanted rice is cultivated, browner where most other crops are grown, and darker under grassland or heavily manured soils.Calcareous brown floodplain soils are extensively developed on the highest part of floodplain ridges of Khulna and Bagherhat Districts rivers.(4) Calcareous Grey Floodplain Soils: Structured, grey silt loams to silty clays, calcareous from the surface or at shallow depths, developed from Gange's alluvium.They become saline in the dry season in the coastal tidal areas.Non-calcareous Dark Grey Floodplain Soils is structured dark grey loamy soil is present in old flood plain ridges and clay basins of the Khulna-Bagherhat area.It is slightly acid to somewhat alkaline in reaction.The basin clays have heavy consistency.Non-calcareous Grey Floodplain Soils is Prismatic and/or blocky structured predominantly grey sandy loams to silty clay loams on young floodplain ridges and silty clay loams in basins, slightly acid to neutral.They become saline in the dry season in the coastal tidal areas.

Figure 1 :
Figure 1: a) Map representing the Bangladesh and its surrounding areas, coastal area of

Figure 2 :
Figure 2: Schematic block diagram showing different steps in the Gamma Spectrometry System health risk brought on by radon exposure and the accumulation of its transient offspring on lung tissues, a quantitative index (Hin) known as the internal hazard index is provided by the equation below(32).

Figure 3 :Figure 4 :Table 3 :
Figure 3: Distribution maps of different parameters in Harbaria area of The Sundarbans mangrove forest.

Figure 1 :
Figure 1: a) Map representing the Bangladesh and its surrounding areas, coastal area of Bangladesh, Sundarbans Mangrove forest and locations of the study areas; b) study areas surrounding geomorphology; c) geomorphology of Karamjal and sampling points; d) geomorphology of Harbaria and sampling points.

Figure
Figure Click here to access/download;Figure;Plos One Figures.docx

Figure 2 :
Figure 2: Schematic block diagram showing different steps in the Gamma Spectrometry System

Figure 3 :
Figure 3: Distribution maps of different parameters in Harbaria area of The Sundarbans mangrove forest.

Figure 4 :
Figure 4: Distribution maps of different parameters in Karamjal area of The Sundarbans mangrove forest.
Natural background radiation is ubiquitous in our dwelling environment.It arises mostly from the Naturally Occurring Radioactive Materials (NORMs) such as 238 U, 232 Th, and40K and their decay products such as 226 Ra, 222 Rn, etc. NORMs are widespread on the Earth's surface and (23)detector was contained in a cylindrical lead shielding device with a sliding cover and a fixed bottom to reduce noise interference from the environment.A detector's ability to differentiate or discriminate the presence of two gamma rays that are closely spaced in energy is defined as energy resolution of the detector.With a relative efficiency of 30%, it was found that the energy resolution of the 1.33 MeV energy peak for 60 Co was 1.67 keV at full-width halfmaximum (FWHM).Using the characteristic gamma lines of 241.98 keV, 295.21 keV and 351.92 keV for214Pb and 609 keV, 1120.3 keV and 1764.5 keV for 214 Bi, the activity concentration of 226 Ra was estimated.On the other hand, the characteristic gamma lines 583.14 keV for 208 Tl, and 911.07 keV and 969.11 keV for 228 Ac, were used to determine the 232 Th activity concentration(21)(22).A weighted mean approach was used to accurately evaluate the radioactivity of 226 Ra and 232 Th.Using the unique 1460.75 keV gamma line, which only occurs individually, the radioactivity of 40 K was estimated.The following equation(23)was used to determine each radionuclide's radioactivity concentration:

Table 1 :
Concentrations of 226 Ra, 232 Th and 40 K in soil samples collected from Karamjal and 45,32t all the values of 40 K (except samples 5 and 6 in Karamjal), very few values of 232 Th (Sample 3 in Karamjal and Sample 14 in Harbaria), and only one value of 226 Ra (Sample 3 in Karamjal) show higher than the population-weighted world average values of 420,45,32

Table 2 :
Radiological data for 238 U, 232 Th,40K associated with soil samples collected from

Table 2
provides a comparative analysis of the mean activity concentrations of 238 U, 232 Th, and40K in the present study with those of other studies analyzing soil samples collected from mangrove forests across the world.The 40 K level was found high in some mangrove forests,